1. Field of the Invention
The present invention relates to a semiconductor device having a plurality of trenches with different opening widths and a method of fabricating the semiconductor device.
2. Description of the Related Art
Semiconductor devices of the above-described type include a semiconductor storage device provided with a number of memory cell transistors, for example. One type of the memory cell transistor is formed with an element isolation region called “shallow trench isolation (STI)” in order that memory cell transistors and peripheral circuit transistors may electrically be isolated. The STI is formed by burying insulating films in a surface of a semiconductor substrate. Each transistor is formed in an active region isolated by the STI.
In a memory cell transistor forming region, a trench of the STI has a smaller width and a smaller depth in order that a degree of integration may be improved. The depth of the trench is rendered smaller in order that the insulating film may desirably be buried in the trench with a small opening width. Furthermore, regarding high-breakdown voltage transistors in the peripheral circuit region, the trench has a larger width and a larger depth than the trench at the memory cell transistor side in order that a prescribed breakdown voltage may be ensured.
In view of these limitations, trenches with different depths in a chip need to be formed in order that the STI may meet the above conditions. For this purpose, a photolithography process has conventionally been carried out twice so that a deep part and a shallow part have been individually formed.
However, carrying out the photolithography process twice is undesirable from the viewpoint of cost reduction. Furthermore, the photolithography process carried out twice results in a stepped portion due to etching in a boundary between the deeper and shallower trenches. A dummy region has conventionally been provided for preventing an element property from being adversely affected by the stepped portion.
JP-A-2000-323564 discloses a technique of carrying out the photolithography process only once even when the trenches with different depths are formed. In the disclosed technique, the photolithography process is carried out only once so that trenches having the same depth are formed in the memory cell region and the peripheral circuit region. A hydrogen heat treatment is carried out in a second half process of the semiconductor manufacture so that the bottom of the narrow trench is raised upward. As a result, the trenches having different depths are formed.
However, the hydrogen heat treatment is required although the photolithography process can be reduced. Accordingly, when a process in which a gate electrode is previously formed and separated by the forming of the element isolation regions, there is a possibility that the previously formed gate oxide film may be damaged by hydrogen anneal, whereupon the film quality may be reduced with the result of reduction in the reliability of elements.